If a model carries a special close combat weapon, he must fight with it in the Close Combat phase – he cannot elect to wield his hand weapon instead. The reason for this is quite straightforward. Almost all warriors train exclusively with one particular weapon, honing their skills of attack and defence with the chosen tool of slaughter. Such warriors are reluctant to surrender advantage to the foe by fighting with a less familiar, or less powerful, weapon. Should the special weapon be destroyed then, and only then, can they use their hand weapon.
The one exception to this rule is that if the model also carries a magical close combat weapon of some kind, he will always use it in preference to other weapons he carries. If the magical weapon is somehow destroyed or rendered useless, the warrior will then use his special close combat weapon (if he has one) finally resorting to his hand weapon if the special close combat weapon is also destroyed or rendered useless. Hand weapons cannot be destroyed (well, they can, but we assume that the warriors are always able to immediately find plenty of replacements among the debris littering the battlefield!). Further details on magic weapons and other items can be found in the Reference section.